After production, substrates are often subjected to downstream processing and handling steps that may generate electrostatic charge (“ESC”). For example, a glass substrate may be subjected to various conveyance and/or positioning processes during which the contact of the substrate with one or more components may cause undesirable ESC build-up on either the surface in contact with the component (“B surface” or “conveyance surface”) and/or the reverse surface (“A surface” or “fabrication surface”) of the substrate. Excessive ESC on either the A surface or the B surface can be undesirable, and potential differences on the A surface of the substrate may pose serious issues during fabrication leading to significant losses in yield, particularly in the case of fabrication of electronic devices, including without limitation fabrication of thin film electronic devices as may be employed during the manufacture of display devices.
During the fabrication process, a conveyance system may be used to transport the substrate from one process station to another. Generally, conveyance systems can comprise a number of small rollers, which may be free-rolling and/or driven. Contact between the B surface of the substrate and the rollers can itself result in ESC build-up on the A and/or B surface. Further, if one or more roller is moving at a different speed than the rest of the rollers in the conveyance system, e.g., a free-rolling or driven roller that is not sufficiently lubricated, the ESC build-up can be further increased.
Another ESC generating process can include vacuum processes, e.g., vacuum chucking, during which the substrate is held in place by a vacuum on a contact surface. The pulling of the substrate by the vacuum can impart charge to the substrate through friction between the substrate and the contact surface area surrounding the vacuum port, as well as through intimate contact between the substrate and the contact surface, during which time charge may be exchanged through van der Waals interactions. ESC build-up can also result from contact between the substrate and other surfaces during the fabrication process, e.g., by rubbing and/or friction.
Current methods and apparatuses for measuring and simulating ESC generating activities are limited, either by poor range of movement, the inability to test more than one type of ESC generating activity, and/or by the inability to evaluate ESC generation as a function of location on the substrate surface. One method for generating and measuring ESC on a substrate is the rolling sphere test, in which a circular rolling ball is contacted with a substrate. However, the use of a circular rolling ball may provide a limited motion profile and may not accurately simulate roller conveyance. In particular, the contacting of a stationary substrate with a rolling ball or the contacting of a moving substrate with a stationary roller ball is not substantially similar to an actual roller conveyance process in which a moving substrate contacts a spinning or rotating roller, e.g., when the roller rotational velocity and the substrate translation velocity may be independent of one another. Moreover, the rolling sphere test provides no information with respect to ESC generation due to vacuum lift and/or frictive contact. Finally, the rolling sphere test provides no method by which all or a portion of the surface of the substrate can be mapped and evaluated in terms of ESC build-up.
Accordingly, it would be advantageous to provide an improved methods and apparatuses for generating and measuring ESC on a surface of a substrate. It would also be advantageous to provide methods and apparatuses which can more accurately simulate one or more types of ESC generating activities.